The fungal pathogen Pneumocystis spp. is a major cause of severe lung infection in immunocomprised patients. Using a novel technique to label surface proteins of live Pneumocystis murina, we have identified and squenced an abundant but previously uncharacterized surface peptidase, designated SPD 1. Serum from immunocompetent mice that cleared P. murina infection contained antibodies to SPD 1, and vaccination with the N terminal region of SPD 1 generated IgG antibodies to Pneumocystis. Importantly, vaccination of mice with SPD 1 followed by depletion of CD4+ T-cells was protective against experimental infection with P. murina. These results lead to our hypothesis that a protective immune response to Pneumocystis requires humoral immunity to SPD1. We will test this hypothesis in two Specific Aims. In Specific Aim 1 we will seek to develop a more effective vaccine using SPD 1 protein fragments and will examine the role of memory B-cells in the immune response. Mucosal vaccination strategies will also be used to enhance the host response in lung tissue. In Specific Aim 2 we will develop and characterize monoclonal antibodies to SPD 1 and will use these antibodies in passive vaccination strategies to treat established infection with P. murina. The results of these studies will lead to new information about the humoral immune response in host defense against Pneumocystis and will lead to new therapies to prevent or treat pulmonary infection with this important human pathogen.